lib/gpg-verify: Add an OstreeGpgError error domain

lib/gpg-verify: Add an OstreeGpgError error domain

Add a new error domain for GPG signing/verification errors, and use it
throughout libostree for describing verification errors. This replaces
various uses of G_IO_ERROR_FAILED, and one instance of
G_IO_ERROR_NOT_FOUND (for which some code in ot-builtin-show.c had to be
changed to ensure it was still handled correctly).

The use of a separate error domain allows failures in GPG operations to
be handled separately from network failures (where the summary file
could not be found to be downloaded, for example) or timeouts.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1064
Closes: #1071
Approved by: mbarnes